Disclosed herein is a continuous process for preparing polymer nanodispersions, in embodiments ink-jettable polymer nanodispersions, comprising providing a polymer solution composition comprising a liquid and a polymer dissolved in the liquid; heating the composition to provide a heated polymer solution; directing the heated polymer solution through a continuous tube wherein the continuous tube has a first end for receiving the polymer solution, a continuous flow-through passageway disposed in an ultrasonic heat exchanger, and a second end for discharging a product stream; treating the heated polymer solution as the solution passes through the continuous flow-through passageway disposed in the ultrasonic heat exchanger to form the product stream containing nanometer size particles in a dispersion; optionally, collecting the product stream in a product receiving vessel; and optionally, filtering the product stream.
Semi-conducting inks are typically made in small laboratory batches, for example in batches of from about 10 to about 200 grams, by heating the polymer solution to dissolution and then immersing the solution in a cool bath under ultrasonication to cool to room temperature, precipitate, and form a dispersion. FIG. 1 illustrates generally a prior art system and process 10 for preparing a polymer nanodispersion comprising flowing a heated stream of polymer solution 12 to be processed through a pipe 14 fitted with ultrasound probes 16, 18. The polymer stream 12 is cooled to room temperature under ultrasonication to precipitate and form a product dispersion stream 20. This process is not scalable and has not been demonstrated beyond 250 grams. The small scale is inadequate to meet current and anticipated quantity needs for semi-conducting materials.
Thin film transistors (TFTs) are fundamental components in modern-age electronics, including, for example, sensors, image scanners, and electronic display devices. TFTs are generally composed of a supporting substrate, three electrically conductive electrodes (gate, source and drain electrodes), a channel semiconducting layer, and an electrically insulating gate dielectric layer separating the gate electrode from the semiconducting layer. It is generally desired to make TFTs which have not only much lower manufacturing costs, but also appealing mechanical properties such as being physically compact, lightweight, and flexible. One approach is through organic thin-film transistors (“OTFT”s), wherein one or more components of the TFT includes organic compounds. In particular, some components can be deposited and patterned using inexpensive, well-understood printing technology. Ink jet printing, such as drop on demand printing, is believed to be a very promising method to fabricate OTFTs. Accordingly, a jettable semiconductor ink is required.
Current processes for preparing semiconductor nanoparticles, in embodiments, polythiophene nanoparticles, such as poly(3,3′″dialkylquaterthiophene) (PQT-12) generally comprise three steps. First, the polymer is dissolved in a suitable solvent, such as dichlorobenzene, at sufficient temperature to ensure complete dissolution. Next, the solution is ultrasonicated by immersion of the dissolution vessel in a room temperature or chilled ultrasonic bath for a suitable period of time, typically about 3 minutes, to precipitate polymer nanoparticles. Finally, the resultant polymer nanodispersion is filtered, such as through a 0.7 micrometer pore size glass fiber filter paper.
It is desirable to prepare polymer nanodispersions in larger than laboratory batch quantities. However, there are challenges to scaling up this process. For example, the limited surface area of batch reactors makes it difficult or impossible to achieve the chill rates required to ensure desired ink quality and high yields at larger scales in a batch process. The reactor volume can be increased. However, as reactor volume increases, the surface area to volume ratio decreases resulting in lower cooling capacities. It may be possible to chill/ultrasonicate a 250 milliliter reactor from dissolution at about 60 to about 70° C. in 2 to 3 minutes or less if chilling can be applied to the bath, and 500 grams may be possible, but volumes of 2 liters and more will cool much more slowly than volumes of 200 milliliters. Because particle size will increase as cooling rate decreases, the particle size of the dispersion will be undesirably larger. When the particle size is too large, the dispersion has lower mobility, less dispersion stability, and becomes difficult or impossible to filter through a 0.7 micrometer filter media thereby affecting consistency of the final concentration of polymer in solution. Dispersions prepared using a cooling rate of 5° C./minute with ultrasonication (which by scale-up standards is very fast) do not produce good dispersions. The particle size is large enough that they cannot be filtered through a 0.7 micrometer pressure filter.
There are inherent limits to the scale of the batch process from an ultrasonication and heat transfer perspective. It is difficult or impossible to locate or build an ultrasound bath of sufficient size with sufficient energy input to hold large reactors Immersion type ultrasonic generators can be employed, rather than ultrasonic baths. However, immersion type ultrasonic generators produce dispersions with undesirably large particles. Therefore, the scale up of the process is limited in reactor size to that which can fit in a chilled ultrasound bath (typical volumes are less than 2 liters) in order to achieve desired small particle size.
While known compositions and processes are suitable for their intended purposes, a need remains for an improved method for preparing polymer nanodispersions. What is further needed is a process for preparing polymer nanodispersions that can be scaled up to desired volumes. What is further needed is a process that provides faster cooling to yield smaller high mobility particles and that provides stability to the ink dispersion. What is further needed is a process that provides fast cooling rates sufficient to achieve high yields and a cost effective product. What is still further needed is a process wherein ultrasound energy density can be preserved while enabling larger bath sizes not currently available.
The appropriate components and process aspects of the each of the foregoing U.S. Patents and Patent Publications may be selected for the present disclosure in embodiments thereof. Further, throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.